One conventional bar code reader is described in the report "BAR CODE READER" on pages 120 to 124 in a journal "O plus E" published June, 1990 by Hiroyuki Miyazaki. In this journal, the bar code reader includes a lens system for supplying a laser beam to a bar code. The lens system includes a first lens fixed on the side of an object point and a second lens which is slidable along an optical axis on the side of an image point. The second lens slides with a predetermined stroke to shift a focal point in accordance with a distance to the bar code, so that a depth of reading the bar code can be expanded.
In the lens system, when the laser beam is directed onto the bar code, a reflection light modulated by black and white stripes of the bar code is detected, so that the bar code is read.
According to the conventional lens system, a shift amount of the image point is equal to a product between a moving amount of the object point on the second lens and a longitudinal magnification of the lens system, when the object point or the second lens is moved in an optical axial direction. The longitudinal magnification is equal to the square of a transversal magnification of the lens system, which is an imaging magnification, so that the image point is greatly shifted by moving either the object point or the second lens along an optical axis even to a small extent, when the imaging magnification is set to be large.
However, the conventional lens system has disadvantages in that steps for adjusting a focal point are increased in number, and a beam diameter becomes small and thus picks up noise caused by print irregularity, etc. This is because beam diameters are changed by a factor of two between the nearest image point and the farthest image point, so that a beam diameter of the object point is required to be equal to a beam diameter obtained at a time of the maximum imaging magnification. As a result, a beam diameter on an image point obtained at a time of the minimum imaging magnification is half a predetermined beam diameter, so that the focus depth becomes shallow. Although the shallow focus depth can be overcome by setting the shift amount steps for adjusting a focal point to be small increments, this results in an increase of the focal point adjusting steps.
Otherwise, a zoom lens in which an image point can be moved without changing the beam diameter at the image point is described on pages 270 to 272 in a journal "KOGAKU" of Hiroshi Kubota published by Iwanami Book Store. According to the zoom lens, however, there is a disadvantage in that three lenses are required to be moved, respectively, so that the structure of the system becomes complicated and expensive.